The present disclosure relates to a memory element formed of a memory layer that can store a magnetization state of a ferromagnetic layer as information and a magnetization pinned layer having a pinned magnetization direction, in which current is caused to flow in a direction perpendicular to a film surface to inject spin-polarized electrons, so that a magnetization direction of the memory layer is changed. The present disclosure also relates to a memory including the memory element that may suitably be used as a non-volatile memory.
High-speed and high-density DRAMs have been widely used as random access memories in information devices such as computers.
However, since DRAMs are volatile memories in which information is erased when a power supply is switched off, non-volatile memories in which information is not erased have been desired.
According to Nikkei Electronics, Feb. 12, 2001 (pp. 164-171), for example, Magnetic random access memories (MRAMs) that are configured to record information by magnetization of a magnetic material have attracted attention and developed as potential non-volatile memories.
In an MRAM, current is caused to flow into two types of address wirings almost perpendicular to each other (word lines and bit lines), respectively, to invert magnetization of a magnetic layer of a magnetic memory element in an intersection of the address wirings based on a current magnetic field generated from each address wiring, so that information is recorded.
A schematic view (perspective view) of an MRAM is shown in FIG. 1.
In an area isolated by an element isolation layer 102 of a semiconductor substrate 110 such as a silicon substrate, a drain region 108, source regions 107, and gate electrodes 101 which form selective transistors for selecting each memory cell are respectively formed.
Word lines 105 extending in a longitudinal direction in the figure are provided above the gate electrodes 101.
The drain region 108 is formed both on the left and right selective transistors in the figure, and a wiring 109 is connected to the drain region 108.
Magnetic memory elements 103 each having a memory layer in which a magnetization direction is inverted are placed between the word lines 105 and bit lines 106 that are placed above the word lines 105 and extend in a transverse direction in the figure. The magnetic memory elements 103 are formed of magnetic tunnel junction elements (MTJ elements), for example.
Further, the magnetic memory elements 103 are electrically connected to the source regions 107 through a bypass line 111 in a horizontal direction and a contact layer 104 in a vertical direction.
Current is caused to flow into the word lines 105 and the bit lines 106, respectively, to apply a current magnetic field to the magnetic memory elements 103, so that a magnetization direction of the memory layers of the magnetic memory elements 103 can be inverted to record information.
In order to allow a magnetic memory such as MRAM to stably retain recorded information, a magnetic layer (memory layer) to record information preferably has a certain coercive force.
On the other hand, in order to rewrite the recorded information, a certain amount of current is preferably caused to flow into address wirings.
However, in accordance with reduction in size of an element forming an MRAM, address wirings are thin, and thus a sufficient amount of current may not be caused to flow into the wirings.
Under these circumstances, memories configured to use magnetization inversion by spin injection have been attracted attention as those configured to allow magnetization to be inverted using a smaller amount of current as disclosed in Japanese Patent Application Publication No. 2003-17782, U.S. Pat. No. 6,256,223, Non-Patent Document Phys. Rev. B 54.9353 (1996), and Non-Patent Document J. Magn. Mat. 159.L1 (1996), for example.
In magnetization inversion by spin injection, electrons spin-polarized by passing through a magnetic material are injected into another magnetic material to invert magnetization in the other magnetic material.
For example, current is caused to flow into giant magnetoresistance elements (GMR elements) or magnetic tunnel junction elements (MTJ elements) in a direction perpendicular to a film surface of the elements, so that a magnetization direction of at least some of magnetic layers of the elements can be inverted.
Magnetization inversion by spin injection is advantageous in that magnetization can be inverted without increasing an amount of current even if an element is reduced in size.
FIGS. 2 and 3 show schematic views of a memory configured to utilize the above-described magnetization inversion by spin injection. FIG. 2 is a perspective view, and FIG. 3 is a sectional view.
In an area isolated by an element isolation layer 52 of a semiconductor substrate 60 such as a silicon substrate, a drain region 58, source regions 57, and gate electrodes 51 which form selective transistors for selecting each memory cell are respectively formed. Of these, the gate electrodes 51 also function as word lines extending in a longitudinal direction in FIG. 2.
The drain region 58 is formed in common with the left and right selective transistors in FIG. 2, and a wiring 59 is connected to the drain region 58.
Memory elements 53 each having a memory layer in which a magnetization direction is inverted by spin injection are placed between the source regions 57 and bit lines 56 that are placed above the source regions 57 and extend in a transverse direction in FIG. 2.
The memory elements 53 are formed of magnetic tunnel junction elements (MTJ elements), for example. Reference numerals 61 and 62 in the figure denote magnetic layers. One of the two magnetic layers 61 and 62 is a magnetization pinned layer in which a magnetization direction is pinned, and the other is a magnetization free layer in which a magnetization direction is changed, specifically, a memory layer.
The memory elements 53 are connected to the bit lines 56 and the source regions 57 respectively through upper or lower contact layers 54. Thus, a magnetization direction of the memory layer can be inverted by spin injection by causing current to flow into the memory elements 53.
Such a memory configured to utilize magnetization inversion by spin injection has a feature in that the memory can have a device structure more simplified as compared with a common MRAM shown in FIG. 1.
The memory configured to utilize magnetization inversion by spin injection is more advantageous than a common MRAM in which magnetization is inverted by an external magnetic field, because an amount of writing current is not increased even if the elements are further reduced in size.
In an MRAM, writing wirings (word lines and bit lines) are provided separate from memory elements, and information is written (recorded) based on a current magnetic field generated by causing current to flow into the writing wirings. Thus, an amount of current which may be necessary for writing can be sufficiently caused to flow into the writing wirings.
On the other hand, in a memory configured to utilize magnetization inversion by spin injection, spin injection is preferably performed by causing current to flow into a memory element to invert a magnetization direction of a memory layer.
Since information is written (recorded) by directly causing current to flow into the memory element in this manner, the memory element is connected to a selective transistor to form a memory cell in order to select a memory cell into which data is written. In this case, an amount of current caused to flow into the memory element is limited to an amount of current which can be caused to flow into the selective transistor (saturation current of the selective transistor).
Therefore, writing is preferably performed using current in an amount equal to or smaller than the saturation current of the selective transistor, and an amount of current caused to flow into the memory element is preferably reduced by improving spin injection efficiency.
In order to amplify a read signal, a high magnetoresistance change rate may preferably be obtained. To secure a high magnetoresistance change rate, it is effective to provide a memory element having an intermediate layer in contact with both sides of the memory layer that is a tunnel insulating layer (tunnel barrier layer).
When the tunnel insulating layer is used as an intermediate layer in this manner, an amount of current caused to flow into the memory element is limited in order to prevent dielectric breakdown of the tunnel insulating layer. From this viewpoint, an amount of current during spin injection is preferably suppressed.
Typically, a memory is configured to store and retain information written by current. Thus, a memory layer may need stability against thermal fluctuation (thermal stability).
A memory element utilizing magnetization inversion by spin injection has a memory layer having a smaller volume than that of a memory layer of an MRAM of related art. That is, the memory element tends to have decreased thermal stability.
When the memory layer includes no secured thermal stability, an inverted magnetization direction is inverted again by heat, thereby causing a writing error.
Therefore, thermal stability is a highly important property in the memory element utilizing magnetization inversion by spin injection.
Generally, an element that uses less energy for writing has a lower energy barrier, and hence information may easily be erased from the element.
By contrast, an element that uses more energy for writing may form a higher energy barrier, and hence information may be retained with stability.
When compared memory elements utilizing magnetization inversion by spin injection that are configured to have equal spin injection efficiency, thermal stability increases with an increase in an amount of saturation magnetization and a volume of a memory layer, thereby consuming a larger amount of current for writing.
A thermal stability index may generally be represented by a thermal stability parameter (Δ).
The thermal stability parameter (Δ) is obtained from the following equation:Δ=KV/kT (K: anisotropic energy, V: volume of the memory layer, k: Boltzmann constant, T: temperature).
Accordingly, in order for a memory that is configured to have a memory layer in which a magnetization direction is inverted by spin injection to be used as a memory element, an amount of current necessary for magnetization inversion may be reduced to equal to or smaller than saturation current of a transistor by increasing spin injection efficiency, and thermal stability may be acquired to stably retain written information.